1. Field of the Invention
The present invention relates to an endoscope system for narrowband light observation with a simultaneous type image sensor and a method for operating an endoscope system.
2. Description Related to the Prior Art
It has been common to make diagnoses with the use of endoscope systems in recent medical care. The endoscope system comprises a light source device, an electronic endoscope, and a processor device. The endoscope system is used for normal light observation and narrowband light observation. In the normal light observation, visible light is used for observing an observation object inside a body cavity. In the narrowband light observation, narrowband illumination light is used. In the normal light observation, bloodstream, which represents the state of blood vessels extending in the observation object, is often unclear in the optical information obtained by the use of the visible light. In the narrowband light observation, visual identification of the state of the bloodstream or the like is improved and the state of the bloodstream of surface blood vessels is observed to determine the stage, the invasion depth, and the like of a lesion.
The narrowband light observation is performed in a frame sequential method or a simultaneous method. In the frame sequential method, blue narrowband light with the center wavelength of 415 nm±10 nm at which an extinction coefficient of hemoglobin is high and green narrowband light with the center wavelength of 540 nm±10 nm is alternately applied to the observation object. Every time each narrowband light is applied, a reflection image is captured with a monochrome image sensor. In the simultaneous method disclosed in Japanese Pat. No. 4009626, the blue narrowband light with the center wavelength of 415 nm±10 nm and the green narrowband light with the center wavelength of 540 nm±10 nm is applied simultaneously and the mixed-color light thereof is simultaneously detected with a color image sensor.
In the case where a complementary color image sensor having Cy pixels, Mg pixels, Ye pixels, and G pixels is used as an image sensor in the simultaneous method disclosed in the Japanese Pat. No. 4009626, the sensitivity of the complementary color image sensor is low on the short wavelength side. Hence, a ratio of a signal component corresponding to the blue narrowband light, of an image signal obtained by the image capture, is also low (see FIG. 7). For this reason, the contrast of the surface blood vessels is low.
Of the Cy pixels and the Mg pixels that are sensitive to the blue narrowband light, the Cy pixels are sensitive also to the green narrowband light. Hence, the blue narrowband light cannot be isolated. The Mg pixels are also somewhat sensitive to the green narrowband light so that the blue narrowband light cannot be isolated completely (see FIG. 8). The images in different layers (surface and subsurface layers) are combined and cannot be isolated from each other. As a result, the contrast of the surface blood vessels is low.
As for wavelength transmission properties of a bundle fiber used as a light guiding member of the endoscope, attenuation factor for light on the short wavelength side, which is less than or equal to approximately 440 nm, is high. Accordingly, the transmittance may often be reduced (see FIG. 12). In other words, in the bundle fiber, an amount of the blue narrowband light attenuated is greater than amounts of other types of narrowband light attenuated. Hence, a ratio of the signal component corresponding to the blue narrowband light to the image signal obtained by the image capture is low. As a result, the contrast of the surface blood vessels is low.